Signaling system



2 Sheets-lSheet ,2

LL //v VEN 70H5 YNQ SIGNALING SYSTEM Filed April 5, 1940 AMAA A. W. HORTCN, JR., ETAL QM. 4 w wm E om Oct. 27, 1942.

o. .2 MURPHY H. E. VAUGHAN By A r Tom/EV Patented oct. 27,1942

UNITED *STATES PATENT OFFICE 2,299,822 SIGNALING srs'rnlyi Arthur Horton, Jr., South Orange, N. J., and Orlando J. Murphy, New York,`and` Henry E. Vaughan, Valley Stream, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 5, 1940, Serial No. 327,962 16 Claims. (Cl. 179%84) According to one feature of the invention, a

signal receiver is arranged for responding to signals consisting of current of one or more par-` tisular frequencies followed by current of a different frequency or combination of frequencies, false operation of the signal receiver in response to interference currents which include signal f requency components being prevented by limiting the current of all frequencies to a value sufficient for operating a signal relay only when substan? tially all of the current is of the one or the other of the signaling frequencies or signaling frequenf cy combinations.

Another feature of the invention is a signal receiver of the abovefdesoribed character in which false operation is `further prevented by making u? the receiver non-responsive to current of the second frequency or frequency @Ombneton U11- less this current follows within a definite intereval of time after the current o f the first fre-V quency or frequency combination ends. A,

A further .feature is e Signal .receiver 0f either of the above-described characters in which false operation is further prevented by an interlocking relay arrangement.

A lclear and complete understanding of the inf vention will be facilitated by considering specific circuit arrangements in which the invention is embodied, three such arrangements being shown schematically in the drawings which form a part of this specification.

Referring to the drawings: i

Fig. 1 shows a line LI and a signal receiver comprising a volume limiting amplifier l0, filters and 2|, detectors I2 and 22, signal responsive relays I3, I4, 23 and 24, timing relays |15 and 25, a cut-off relay 20 and signal repeating relays I8 and 28; i

Fig. 2 shows a line L2 and a signal receiver comprising a volume limiting amrlier .3.0, filters relays 33, 34, 43 and 44, timing relays 35 and 45, a cut-off relay 40 and signal repeating relays 38 and 48; and

Fig. 3 shows a line L3, a filter F, transformer v TI, an amplifier AI, a transformer T2, a volume limiting amplifier A2, filters Fl and F2, rectiners RI and R2, signal receivingrelays 53 and E3, timing relays 54 and 64, and signal repeating relays 5T and 61.

Each of the signal receiving devices shown in Figs. 1, 2 and 3 is particularly adapted to respond to voice frequency signals comprising current of 3| and 4I, detectors 32 and 42, signal responsive 55 two frequencies in sequence. Signal operation is effected by a current of one of the two signal frequencies followed within a short and predetermined interval of time by current of the other frequency in the line. Due to the use of a Volume limiting amplifier, there is no signal operation in response to current comprising more than one of the signal frequencies'at a time. The character of thesignals andthe sequence of the'cur.-

` rents of the two frequencies are as follows:

Seizure signal-Current of f2 followed by current of f1. Dial impulses andV rerin'g signal-Current of f1 followed bycurrent of f2. Supervisory signal:

` 'Off hook- Current of f2 followed by curr-ent of f1. v On hook-Current of f1 followed by current Of f2. i Disconnectsignal-Current of f1 followed by a long application of current of f2.

' Consider first the arrangement shown in Fig. 1, in which all current in line LI, signaling current, voice current, and noise or other interference current, is amplified by the volume limiting ampliner 0 which may be similar to any one of those disclosed in the patent to G. T. Lor-- ance No. 1,654,085, granted December 27, 1927, or to any one o f those ydisclosed in the patent to E. Peterson No. 1,830,240, granted November 3,

, 1931. The output of the amplifier Iii is limited to a value which is slightly in excess of that required for operating each of the signal receiving relaysu I3 andY 23. The two signal frequencies f1 and f2 are Within the voice frequency range and may, for instance, be 1100 cycles and 1750 cycles. The output of the amplifier l0 is connected to the input side of two filters Il and 2|, which are designed so that one passes current of one of the two signaling frequencies, and the other passes current'of .the other of these frequencies.

The currents, Passed by ltere l! and i2' are am# plified and rectified by amplifier-detectors I2 and 2.. which may be of well-known thermionic type.

Assume now that the line LI is seized at its distant end and that responsive to seizure a source of current of frequency f2 is connected to the line for an interval long enough toeffect the operation of relay 23 and that a source of current of frequency f1 is connected to the line within a short and predetermined interval after the source of frequency f2 is disconnected. The current of frequency f2 is amplified by the volume limiting amplifier II). There being no other frequencies present, the amplified current of frequency f2 passes through filter 2|, is rectified by detector 22, and effects the operation of relay 23;

but relay 24 does not operate because its winding'is short-circuited through a back contact of relay I5. Relay 23 closes circuits including a back contact of relay I3 for operating relay 25 and short-circuiting condenser 26. While relay 23 is operated it maintains a short 'circuit across the winding of relay I4, although the normally closed short circuit is opened by the operation of relay 25. When the current of frequency f2 ceases, relay 23 releases, opening the operating circuit of relay 25; but relay 25 is held operated for a short and predetermined interval of time by current charging condenser 26. Before relay 25 releases, the current of frequency f1 in line LI causes the operation of relays I3 and I4. Relay I3 closes circuits including a back contact of relay 23 for operating relay I5 and short-circuiting condenser I6. Relay I4 opens the circuit through resistor I9 and winding of relay 20 to reduce the current through the winding of normally operated relay 26 to effect its release. It is the function of relay 20 to disconnect from line LI any other line or trunk to which a connection has been extended while .signals are being received over line LI, so that the signals will not be further transmitted over the connected line or trunk. Relay I4 also closes a circuit for operatively energizing the upper winding of relay I8. Relay I8 locks through its lower winding and closes a circuit including the back contact of relay 28 through conductors I and 2, this closure constituting response to the seizure signal consisting of a current of frequency fz followed within the holding time of relay 25 by current of frequency f1. The circuit through conductcrs I and 2 may lead to the line relay of a selector switch or to a register, depending on the type of switching control employed.

The signaling current of frequency f1 may be maintained until dialing begins or may be ended as soon as the operation of relays I3 and I4 has effected the operation of relay I8. If the application of current of frequency f1 terminates before dialing begins, relays I3 and I4 release. The release of relay I3 opens the circuit for operating relay I5, the release of relay I being delayed for a predetermined interval of time while condenser I6 is being charged.

The dial and impulse transmitting apparatus at the distant end of line LI may be arranged so that the source of current of frequency f1 is applied to the line when the dial is moved olf normal to prepare for the transmission of dial impulses, each opening of the impulse contacts of the dial effecting the disconnection of the source of frequency f1 and the connection of the source of frequency f2 to the line LI, and each closure of the impulse contacts effecting the disconnection -of the source of frequency f2 and reconnection of the source of frequency f1 to the line.

The current of frequency f1, occurring after the seizure signal is ended, causes the operation of relay I3; but relay I4 does not respond because its winding is short-circuited. Relay I3 closes the circuit for operating relay I5 and maintains a short circuit across the winding of relay 24 after relay I5 operates. When the cul'- rent of frequency f1 ends at the beginning of the -rst dial impulse, relay I3 releases; and alter condenser I6 is charged, relay I5 releases. The impulse of current of frequency f2, which follows the current of frequency f1 before relay I5 releases, causes the operation of relays 23 and 24.

Relay 23 closes the circuit for operating relay 25. Relay 24 closes a circuit for operating relay 28 and opens the circuit through resistor 29 to effect the release of relay 28, the release of relay 20 occurring before relay 28 operates. Relay 28 opens the circuit through conductors I and 2, this Aopening constituting the beginning of the response to the first dialimpulse. Relay 24 also opens the locking circuit of relay I8, but relay I8 is slow in releasing and the openings of its locking circuit during the receipt of dial impulses are not long enough to cause the release of relay I8. When the impulse of current of frequency f2 end, relays 23 and 24 release. The release of relay 24 causes the release of relay 28 and the release of relay 28 ends the opening of the impulse receiving circuit through conductors I and 2, thus completing the response to the first dial impulse. The current of frequency f1, which follows the current ofl frequency f2 before relay 25 releases, effects the operation of relay IS and also the operation of relay I4. The operation of relay I4 opens the circuit through resistor I9 to prevent the reoperation of relay 28 and closes the operating circuit through the upper winding of relay I 8. Each succeeding dial impulse, transmitted over` line LI, effects an opening of the impulse receiving circuit through conductors I and 2 in the same manner. When all of the dial impulses in a train have been received, current of frequency f1 effects the operation of relays I3, I4 and I5. When the current of frequency f1 ceases, relays I3, I4 and I5 release and relay 28 reoperates; but relay I8 is held operated by its locking winding.

Additional trains of impulses, if any, transmitted over line LI to complete a desired connection, are received in like manner. The transmission of a ringing signal, which signal consists of a sequence of currents similar to that transmitted for a single dial impulse, effects an operation of the signal receiver to open and close the circuit through conductors I and 2. The release of relay 20, during response to a ringing signal, disconnects a connected outgoing trunk or line.

A disconnect signal consists of current of frequency fi followed by current of frequency f2 for an interval of time long enough to effect the release of relay I8. Relays I3 and I5 are operated and released successively in response to the current of frequency f1 and relays 23 and 24 are actuated by the current of frequency f2 received before relay I5 releases. Relay 28 operates to open the circuit through conductors I and 2 and the operation of relays 23 and 24 is continued for a long enough interval to effect the vrelease of relay I8 and thus prevent the reclosure of the circuit through conductors I and 2 when relay 28 releases. The opening of the circuit through conductors I and 2 effects the release of any partial-ly or completely established connection.

On a call outgoing over line LI, supervisory signals will be received. An answering supervisory, or off-hook, signal consists of current of frequency fz followed by current of frequency f1, the same as a seizure signal; and the closure of the circuit through conductors` I and 2` is effected responsive to this sequence of signaling currents in the manner hereinbefore described. If the called subscriber replaces the receiver, an on-hook supervisory signal is received over line LI, constituting current of frequency f1 followed by current of frequency f2, the same as for a dial impulse, and the opening and reclosure of the circuit through conductors I and 2 is effected in response to this signal in the manner hereinbefore described. When the connection is released at the calling end, the locking circuit of relay I8 is opened by means not shown in the drawings to restore the signal receiver to normal.

Consider next the signal receiver shown in Fig. 2 which is arranged so as to respond to dial impulses, each consisting of current of frequency f1 followed by current of frequency fz. The volurne limiter 30, filters 3| and 4I, amplifier-detectors 32 and 42relays 33, 34, 43, 44 and 38, and condensers 36 and 46 are similar in function and operation to like elements of the signal receiver shown in Fig. 1. The cut-off relay 40, which is similar in function to the cut-off relay 20 of Fig. 1, is a normally released relay which is operated when either of relays 34 or 44 is operated and which remains operated after its operating circuit is opened, for a predetermined interval of time, while condenser 39 is being charged. This interval is long enough to prevent the release of relay 40 during response to a train of dial impulses. The operation of relay 4I), in response to an operation of relay 34, is effected just before relay 38 operates. The condenser 49, connected in series with the winding of relay 48, is charged when the operating circuit of relay 48 is opened, thereby `holding relay 48 for a predetermined interval of time to prevent distortion in repeating a train of dial impulses.

Upon transmission of the seizure signal, relay 43 responds to thecurrent of frequency f1, closing the circuit for operating relay 45. At the end of this current, relay 43 releases, opening the circuit through the winding of relay 45. Relay 45 is held operated for a predetermined n-` terval by the current charging condenser 46. The current of frequency f2, following the current of frequency ,f1 before relay 45 releases, effects the operation of both of relays 33 and 34. Relay 33 operates relay 35. Relay 34 operates the cut-off Arelay 40 and operates relay 38. Relay 38 locks through its lowerA winding and closes the signal responsive circuit through conductors 3 and 4. Upon transmission of dial iinpulses, current of frequency f1 effects the operation of relays 33 and 35; and current of frequency f2, following the current of frequency f1 before relay 35 releases, effects the operation of relays 43, 44, 45 and 40. After relay 4D has operated, relay 48 operates, opening the signal responsive circuit through conductors 3 and 4. At the end of the current of frequency f2relays 43 and 44 release and, after condenser 46 is charged, relay 45 releases. The release of relay 44 opens the circuit through the Winding of relay 48; and, when condenser 49 is charged, relay 48` releases, reclosing the circuit through con-` ductors 3 and 4.A The interval during which the release of relay 48 is delayed, by the chargingof condenser 49, is equal to the interval during which current of frequency f1 is transmitted for each dial impulse so that the duration of the' opening of the circuit through conductors 3 and 4 is of the same duration as a dial impulse and there is, therefore, no distortion of the dial impulses in the signal receiver.

The signal receiver of Fig. 2 responds to ringing and supervisory signals in similar manner to that in which it responds to seizure and to dial impulses. Response to the disconnect signal is similar to response to a dial impulse except that the prolonged current of frequency f2 holds relay 44 operated long enough to cause the release of relay 38.

Consider now the signal receiver shown in Fig. 3j of the drawings which includes a filter F which is connected between the line L3 `and the input transformer TI. This lter is tuned to pass currents of the incoming signaling frequencies but not to pass current of the frequencies used for transmitting signals outgoing over the line L2, the apparatus for transmitting such outgoing signals not being shown in the drawings. This signal receiver is arranged to respond to signals of current of one of two signaling frequencies followed within a definite interval of time by current of the other signaling frequency. All current in line LI except that of the frequencies used for outgoing signals is passed through filter F and transformer- TI and is amplified by the triode AI. The output circuit of tube AI includes the left winding of transformer T2, the voltage induced in the rightwinding` being applied through a high resistance element 50, in parallel with a condenser, to` the grid of triode A2. Due to the high resistance of element 50, the current in the output circuit of tube A2 is limited to a value which is` not suflicient to effect the operation. of relay 53 or relay 63 unless substantially all of this current is ofthe one or the other of the two signaling frequencies. The coil 5I and condenser 52 constitute a filter which offers substantially infinite impedance to current of signaling frequency f1, so that current of this frequency in the output of tube A2 is rectified by rectifier RI to energize the winding of relay 53. The coil 6I and condenser 82 constitute a filter which offers subv stantially infinite impedance to current of signaling frequency f2, so that current of this frequency in the output of tube A2 is rectified by rectifier R2 `to energize the winding of relay 63.

To condition the signal receiver for operation, key KI is operated, thereby connecting the filament current source BI and the plate current source B2 to the tubes AI and A2. The source B2 is also connected to conductor 10, thereby effecting the energization of the right, biasing windings of each of relays 53, 54, 51, 63, 64 and 61.

The bias of relays 53, 54, 83 and 64 is such as to l make these relays quickly responsive to current in their operating windings. The bias normally introduced by the energization of the right windings of relays 5'I and 61 is sufficient to prevent their operation when relays 53 and 63 operate in response to voice or other interfering currents of frequencies f1 and fz respectively.

Assume, first, transmission of a signal through line L3 consisting of current of frequency f1 followed by current of frequency f2. The operation of relay 53 in response to the current of fre-l quency ,f1 closes circuits for energizing the left windings of relays 54 and 51. Relay 54 operates but the energization of the left winding of relay 51 is not sufficient to overcome the biasing winding and, therefore, relay 51 does not operate. Relay 54 opens the circuit through resistor 69, thereby decreasing the current in the biasing winding of relay 61. When the current of frequency f1 ceases, relay 53 releases, opening the circuit for energizing the left windings of relays 54 and 51. Relay 54 does not release immediately, being held operated for a predetermined interval of time by the current charging condenser 55. The current of frequency f2, following the current of frequency f1 before relay 54 releases, causes the operation of relay 63 and relay 63 closes circuits for energizing the left windings of relays 64 and 61. Relay 64 operates and, since relay 54 has not released, the energization of the biasing winding of relay 61 is not sufficient to prevent the operation of relay 61 and, therefore, relay 61 also operates. Relay 61 connects ground to conductor 6, thereby effecting operation of signaling means not shown in the drawings. Relay 61 prevents an increase in the energization of its biasing winding when relay 54 releases. The release of relay 63, when current of frequency f2 ends, causes the release of relays 64 and 61. Relay 64 is held operated, for a predetermined interval of time after relay 63 releases, while condenser 65 is being charged. When relay 61 releases, the signal responsive circuit through conductor 6 is opened.

If a signal is transmitted through line L3 consisting of current of frequency f2 followed by current of frequency f1, relay 63 is operated by the current of frequency f2. Relay 63 closes circuits for energizing the left windings of relaysr64 and 61. Relay 64 operates but relay 61 does not, due to the energization of its biasing winding. Relay 64 opens the circuit through resistor 59, thereby decreasing the current through the biasing winding of relay 51. When the current of frequency f2 ceases, relay 63 releases. Relay 64 remains operated, for a predetermined interval of time after relay 63 releases, while condenser 65 is being charged. The current of frequency f1, following the current of frequency f2, now causes the operaiton of relay 53. Relay 53 closes the circuits for energizing the left windings of relays 54 and 51; and, since relay 64 has not yet released, both of relays 54 and 51 are operated. Relay 51 connects ground to conductor 5, thereby effecting operation of signaling means not shown in the drawings. Relay 51 prevents an increase of the energization of its biasing winding when relay 64 releases. The release of relay 53, when current of frequency f1 ends, causes the release of relay 54 and 51, relay 54 remaining operated until condenser 55 is charged.

In each of the arrangements shown in Figs. 1, 2 and 3 the operation of only one of the two signal responsive relays, I3 and 23, 33 and 43, and 53 and 63, is possible at a time because the current output of the Volume limiting amplifier is insucient to effect the operation of any one of these signal relays unless substantially all of the output consists of one only of the two signaling frequencies. Thus the output volume may eX- ceed the minimum operating Value by only 3 or 4 decibels. The closer this limiting volume approaches the minimum operating value, the less chance there is of false operation by voice or interference currents. Furthermore, a signal response cannot be effected unless the current of the second frequency follows the current of the first frequency within a short and predetermined interval of time, which interval is measured by the charging of a condenser in each of the arrangements shown although any other known method of timing this interval may be employed.

What is claimed is:

1. In a telephone system, a line, a signal receiver connected to said line for responding to signals each of which consists of a current of a first predetermined frequency characteristic in said line followed by a current of a second predetermined frequency characteristic, said receiver comprising two relays, one for operation in response to that part of a signal which consists of current of said first characteristic, the other for operation in response to that part of a signal which consists of current of said second characteristic, a filter for selectively passing from said line to said one of said relays current of said rst characteristic, a filter for selectively passing from said line to said other relay current of said second characteristic, and means for limiting the total current input to said relays to a value sufficient for operating only one of said relays at a time.

2. In a telephone system, a line, a signal receiver connected to said line for responding to signals consisting of current of a first predetermined frequency characteristic in said line followed by current of a second predetermined frequency characteristic, said receiver comprising relays for operation by said signaling currents, filters for selectively passing to each of said relays current of a .different one of said frequency characteristics, one of said relays connected for operation in response to that part of a signal which consists of current of said first characteristic and another of said relays connected for operation in response to that part of a signal which consists of current of said second characteristic, and means for limiting the input to said filters to a value sufficient for effecting the operation of a relay only if substantially all of said input is of the frequency characteristic passed to said relay.

3. In a telephone system, a line, a signal receiver connected to said line for responding to signals consisting of current of a first predetermined frequency characteristic in said line followed by current of a second predetermined frequency characteristic, and for responding to signals consisting of current of said second frequency characteristic followed by current of said first frequency characteristic, said receiver comf' prising relays for operation by said signaling currents and filters for selectively passing to each of said relays current of a different one of said frequency characteristics, one of said relays connected for operation in response to that part of a signal which consists of current of said first characteristic and another of said relays connected for operation in response to that part of a signal which consists of current of said second characteristic, and means for limiting the input to said filters to a value sufcient for effecting the operation of a relay only if substantially all of said input is of the frequency characteristic passed to said relay.

4. A signal device adapted to respond to signals consisting of current of a first voice frequency characteristic followed, within a predetermined interval of time after this current ends, by current of a second voice frequency characteristic, said device comprising two signal receiving relays, one for operation responsive to current of said rst characteristic and the other for operation responsive to current of said second characteristic,`lters for selectively passing from said line to each of said relays current of a different one of said characteristics,y meansfor limiting the totalinput to said lters to an amount suicient for operating only one of said relays at a time, a signal circuit, and auxiliary relays controlling said circuit, one of said auxiliary relays being operative in response to the operation of one of said receiving relays followed within a predetermined interval of time after the release of said one of the receiving relays by the operation of the other of said receiving relays, the other of said auxiliary relays being operative inv response to the operation of said-other of the receiving relays followed within a predetermined interval of time after the release of said other receiving relay by the operation of said one of the receiving relays. i l

5. In a signaling system, a line, a signal receiver connected to said line for responding to signals each of which consists of a current in said line of a first predetermined frequency characteristic followed within `a predetermined interval of time by a current of a `second predetermined frequency characteristic, said receiver comprising a volume limiting amplifier the input side of which' is connected to said line, two tuned circuits connected in series in the output circuit of said amplifier, one of said tuned circuits constituting a high impedance to signaling current of said rst characteristic, the other of said tuned circuits constituting a high impedance to signaling current of said second characteristic,

a first relay, a second relay, a rectifier connecting said first relay to said first tuned circuits for operation in response to that part of a signal which consists of current of said first characteristic and arectier connecting said second relay i.

to said second tuned circuit` for operation in response to that part of a signal which consists of current of said second characteristic, the output of said amplifier being only sufficient to effect the operation of one of said relays at a time.r

6. A signal receiver for response to signals consisting of current of a first frequency characteristic followed within a definite interval of time by current of a second frequency characteristic, said receiver comprising relays for operation by said signaling currents, filters for passing to each of said relays current of a different one of said characteristics, a first relay means, said first means responsive to the operation of one of said relays by current of the first characteristic, a second relay means, said second means responsive to the operation 'of the other of said relays by current of said second characteristic, means including said first relay means for preventing the operation of said second relay means unless the operation of said other relay occurs within a predetermined interval of time after the release of said one relay, and means for limiting the input to said filters to a value sufficient for effecting the operation of one of said relays only if substantially all of the input is of the one or the other cf said characteristics.

7. In a signal receiver, according to claim 6, means comprising a condenser fixing said predetermined interval of time.

8. In a telephone system, a line, a signal receiver connected to said line, said receiver arranged for response to signals consisting of current of a first frequency characteristic followed within a definite interval of time by current of a second frequency characteristic, said receiver `comprising signal relays for operation by said `signaling currents, filters for passing to each of put of said amplifier being limited to prevent the operation of anyy of said signal relays unless substantially all of the current in said line is of the one or the other of said characteristics, auxiliary relays, a first auxiliary relay operatively responsive to the operationrof one of said signal relays,

`ayse'cond auxiliary relay operatively responsive to the operation of the other of said signal relays,

Ymeans for holding operated said first auxiliary relay for a predetermined interval of time after said one of theA signal relay releases, and relay means responsive to Signaling current of said `other characteristic only if it occurs before said -rst auxiliary` relay releases.

9'. In a :telephone system, a line, a signal receiver connected to said line, said receiver arranged forresponse .to signals consisting of current of a first frequency characteristic followed within a definite interval Vof time by current of a second frequency characteristic, and to signals consisting of a current of said second frequency characteristic followed within a definite interval of time by current of said first frequency-characteristic, saidreceiver Acomprising signal relays for operation byrsaid signaling currents, filters for passing to each `of saidrelays current of a different one .of said` characteristics, means comprising an amplifier for amplifying all current in said line, the output `of said amplifier being limited to prevent the operation of any of said signal relays Vunless substantially all of the current in said line is of .the one or fthe other of Vsaid characteristics, .auxiliary relays, a first auxiliary relay -operatively .responsive to the operation of a first one of said signal relays, a second auxiliary relay operatively responsive to the operation of a second one of said signal relays, means for holding operated said first auxiliary relay for a predetermined interval of time after said first signal relay releases, a third one of said signal relays responsive to signaling current of said other characteristic only if it occurs before said rst auxiliary relay releases, means for holding operated said second auxiliary relay for a predetermined interval of time after said second signal relay releases, and a fourth one of said signal relays responsive to signaling current of said one characteristic only if it occurs before said second auxiliary relay releases.

10. In a telephone system, a line, a signal receiver arranged to respond to signals consisting of current of a first frequency characteristic in said line followed within a definite' interval of time by current of a secondvfrequency characcurrent of said second frequency characteristic followed within a definite interval of time bycurrent of said first frequency characteristic, signal relays for operation by said signaling currents, lters for passing to each of said relays current of a different one of said characteristics, means comprising an amplifier for amplifying all current in said line, the output of said amplier being limited to prevent the operation of any of said signal relays unless substantially all of the current in said line is of the one or the other of said characteristics, auxiliary relays, a first auxiliary relay operatively responsive to the operation of a rst one of said signal relays, a second,

auxiliary relay operatively responsive to the operation of a second one of said signal relays, means for holding operated said first auxiliary relay for a predetermined interval of time after said first signal relay releases, means for holding operated said second auxiliary relay for a predetermined interval of time after said second signal relay releases, means comprising said second auxiliary relay for rendering a third one of said signal relays responsive to current of said rst frequency characteristic, means comprising said first auxiliary relay for rendering a fourth one of said signal relays responsive to current of said second frequency characteristic, a signal circuit, means responsive to the operation of said third signal relay for closing said signal circuit, and means responsive to the operation of said fourth signal relay for opening said signal circuit.

l1. In a telephone system, a line, signal relays, a first filter and detector for passing` to the operating Winding of a rst one of said relays current of a first frequency characteristic, a second filter and detector for passing to the operating Winding of a second one of said relays current of a second frequency characteristic, means for amplifying all current in said line, the output of said amplifying means being connected to said filters, means for limiting said output to a value sufficient for operating a signal relay only if sub- I stantially all of the current in said line is of the rst frequency characteristic or if substantially all of the current in said line is of the second frequency characteristic, auxiliary relays each having an operating Winding and a biasing Winding, a first auxiliary relay operatively responsive to the operation of said first signal relay, a second auxiliary relay operatively responsive to the operation of said second signal relay, a third auxiliary relay normally biased to prevent its operation responsive to the operation of said rst signal relay, a fourth auxiliary relay normally biased to prevent its operation responsive to the operation of said second signal relay, means for .holding said rst auxiliary relay operated for a predetermined interval of time after said first signal relay releases, means for holding said second auxiliary relay operated for a predetermined interval of time after said second signal relay releases, the operation of said rst auxiliary relay being effective to reduce the bias of said fourth auxiliary relay to render it responsive to the operation of said second signal relay, the operation of said second auxiliary relay being effective to reduce the bias of said third auxiliary relay to render it responsive to the operation of said first signal relay, and signal circuits controlled by said third and fourth auxiliary relays.

12. In a signal device according to claim 6, a control relay responsive to the operation of said Asecond relay means.

13. In a telephone system and signal receiver according to claim 9, a control relay responsive to the operation of each of said third and fourth signal relays.

14. In a telephone system according to claim 10, a control relay responsive to the operation of 'each of said third and fourth signal relays.

15. In a telephone system and signal receiver `according to claim 9, a control relay, a circuit closed by the operation of said third signal relay for operating said control relay, and a circuit closed by the operation of said fourth signal relay for operating said control relay.

16. In a telephone system according to claim 10, a control relay, the energization of the Winding of said control relay being controlled by a contact of said third signal relay and by a contact of said fourth signal relay, said contacts of said third and fourth signal relays being connected in parallel.

ARTHUR W. HORTON, JR. ORLANDO J. MURPHY. HENRY E. VAUGHAN. 

